Method and Device for Encoding and Reconstructing Computer-Generated Video Holograms

ABSTRACT

A method and a device for encoding and reconstructing computer-generated video holograms using a conventional LC display: it provides holographic reconstruction of three-dimensional scenes using electronically controllable pixel in a holographic array ( 3 ) with a conventional resolution, and is reasonably free from flickering and cross-talk. Reconstruction is in real time, and for both eyes at the same time, over a large viewing zone. The method takes advantage of an optical focusing means ( 2 ) in order to image vertically coherent light emitted by a line light source ( 1 ) into viewing windows ( 8 R,  8 L) after modulation by the pixel array ( 3 ). The holographic reconstruction ( 11 ) of the scene is rendered visible from viewing windows ( 8 R,  8 L) for both eyes of an observer by way of diffraction at the pixels. According to the invention, the controllable pixels are disposed in vertical pixel columns ( 15, 16 ), which encode separate holograms of the same scene for each of the viewer&#39;s eyes (R, L), where said holograms are one-dimensional in the vertical direction and horizontally interleaved. An image separation means ( 7 ) with separating elements arranged parallel to the pixel columns reveals the respective pixel columns ( 15, 15 ′ or  16, 16 ′) for one eye and covers them for the other eye.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of German application DE 10 2004044 111.1 filed on Sep. 8, 2004, the entire contents of which is herebyincorporated in total by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and device for encoding andreconstructing computer-generated large-area video holograms using adisplay with conventional (i.e. commercially available) resolution; thedisplay gives a large viewing angle and a high spatial image quality.The display includes a holographic array with controllable pixels, whichelectronically affect the amplitude and/or phase of light. Such an arrayis known as spatial light modulator (SLM). A suitable array for spatialamplitude modulation of a light pattern to reconstruct video hologramsis, for example a Liquid Crystal Display (LCD). However, this inventioncan also be applied to other controllable holographic arrays, which usecoherent light for modulating a light wave front.

2. Definition of Terms

The term “pitch” describes in this document the distance between thecenters of two adjacent pixels of an array. It thus characterizes thedisplay resolution.

The term “encoding” describes the way in which the holographic array issupplied with control values so that it reconstructs a three-dimensionalscene. According to this invention, the scene is viewable through‘viewing windows’.

A viewing window is the intersection of a viewing zone and an observerplane. The observer can see the reconstructed object if at least one eyeis inside the viewing window.

3. Description of Related Art

A drawback of 3D-autostereoscopic displays using conventional optics isa mismatch between parallax information and the accommodation of thelens of the eye. On the one hand, each eye sees a different perspectiveview of a scene, which simulates a depth impression of objects at anarbitrary distance. On the other hand, each perspective view is locatedon the display surface itself. Hence, the eye focuses on the displaysurface, and each eye sees a flat image. This causes a mismatch betweenseeing objects at arbitrary depth (i.e. not on the display surface)achieved by parallax information and the accommodation of the eyes tothe fixed display surface. This may cause an unpleasant feeling and eyefatigue. This can be avoided by using holographic displays, whichreconstruct the objects of a 3D scene at correct depths.

A holographic display reconstructs objects by coherent superposition oflight waves. For this purpose, a spatial light modulator (SLM) generatesa wave pattern. This hologram is the Fresnel transform of the 3D scenewhich is to be reconstructed. The SLM diffracts the light waves andreconstructs the scene. As the hologram is sampled, a periodicreplication occurs that is associated with a periodicity interval. Thus,the observer within a viewing zone given by the periodicity interval cansee the reconstruction. The maximum diffraction angle of the SLM, whichbasically depends on the pixel pitch, determines the viewing zone. Amajor problem in encoding and reconstructing video holograms is that asufficiently large viewing zone must be provided for viewing thereconstruction.

In conventional holographic displays, the viewing zone should cover atleast the eye separation, which requires a pixel size of ca. 10 μm atmost. Even for a small display area of 100 mm*100 mm, the pixel countwill be of the order of 100 million. This causes costly hardware andlong computation times even when the display is simplified to ahorizontal-parallax only hologram. Currently available large-areadisplays typically use holographic arrays with a pixel pitch which onlydiffracts light into a very small viewing zone, so that it is impossibleto view a reconstructed three-dimensional scene with both eyes.

Several solutions are known to solve these problems.

The document, K. Maeno, N. Fukaya, O. Nishikawal, “Electro-holographicdisplay using 15 Mega pixels LCD”, Advanced 3D TelecommunicationProject, 1996, SPIE, Vol. 2652, refers to holographic 3D displays usingcommercially available Liquid Crystal Displays (LCD). This documentdescribes a video hologram-reconstructing device that uses five specialdisplays with a high resolution (instead of a conventional LCD with alow resolution); the viewing zone is enlarged because the resolutionvalues of each display is combined to give an overall high resolution.All displays are joined either directly or by way of opticalreproduction. Only horizontal parallax is used; vertical parallax isdisregarded. The known solution requires a resolution of 15 Mega pixelsprovided in a unit of the five special displays: each has 3,200×960pixels to reconstruct a video hologram in a volume of only 50 mm×150 mmand a depth of 50 mm. The viewing zone is only 65 mm wide, correspondingto about the eye to eye separation distance, so that the scene can onlyjust be viewed with both eyes. The required resolution depends on thedesired size of the video hologram and the viewing zone. However, thisarrangement has significant disadvantages: use of multiple displays anda large lens for reconstruction, leading to a large depth and largevolume, and heavy demands on computing power.

Another way of enlarging the viewing zone is described in the documentby T. Mishina, M Okui, F. Okano, “Viewing zone enlargement method forsampled hologram that uses high-order diffraction”, Applied Optics,2002, Vol. 41, No. 8. According to this method, not only the firstdiffraction order is used for hologram reconstruction, but also furtherdiffraction orders; these are combined to form a common viewing space.The corresponding video holograms for a certain object are shownsequentially on a LC display. With the help of a second LC display,which acts as a spatial frequency filter, the individual diffractionorders are filtered during reconstruction. The visible areas aregenerated sequentially and joined spatially. The achievable viewing zoneis still narrower than 65 mm, so that the reconstructed object can onlybe viewed with one eye. Again, this method has the disadvantage of heavydemands on computing power. In addition, the pixel arrays are requiredto have extremely short switching delays.

When joining several diffraction orders sequentially, the displays usedmust have a high resolution and a high switching speed to prevent theimage from flickering. This is why binary holograms are often used.However, they suffer from substantial errors caused by binary encoding.

A common additional disadvantage of the cited known holographic methodsis the heavy demand on computing power for encoding the holograms.

A device described in Document WO 2003/021363 (A1) for reconstructingcomputer-generated holograms contains a vertically oriented line lightsource that generates holograms with horizontal parallax only. The linelight source generates monochromatic light with a bandwidth of less than10 nm and which is coherent in the horizontal direction but incoherentin the vertical direction.

In conventional holographic displays, the viewing window is much largerthan the pupil of the eye is. A consequence is much effort is done toproject light into regions where no observer is located.

A basic idea of applicant's former patent application WO 2004/044659(A2) is to reduce the viewing window to a size that is just slightlylarger than the pupil of an eye. This will significantly lessen therequirements on the maximum pixel size. The document describes a devicefor reconstructing video holograms in a reduced size viewing window. Thedevice contains at least one point light source or line light source(which provides sufficiently coherent light), a lens, and a holographicarray with cells arranged in a matrix with at least one opening percell, the phase, or amplitude of the opening being controllable. Aviewing plane is located in the image plane of the light source.

The hologram information is sampled in pixels and displayed on an LCDarray. Sampled holograms always have the property of periodicrepetitions of the reconstructed scene and the viewing window. Care hasto be taken that the viewing windows do not overlap, as in that casemultiple reconstructions would be seen. Limiting the area on thehologram on which the scene information is encoded can avoid an overlap.This area has to be limited such that light emanating from reconstructedscene points is confined to one viewing window. Therefore, the devicereconstructs a video hologram in one periodicity interval of the Fouriertransform in a viewing plane. The reconstructed three-dimensional scenecan be viewed with both eyes through a viewing window located in frontof the eyes. The reconstructed scene is visible inside a reconstructionfrustum, which stretches between the display area and the viewingwindow; the scene can thereby be reconstructed on, in front of or behindthe array surface. The known solution allows the use of a conventionalarray with resolution near 3 million pixels at reasonable hardwareexpenses and computing power.

A light source according to this document is considered sufficientlycoherent if the light is spatially coherent to an extent that it allowsinterference, so that it is at least suitable for a one-dimensionalholographic reconstruction with an adequate resolution. Theserequirements can also be met by conventional light sources, like an LEDarrangement, if they radiate light through an adequately narrow gap. Thespectral bandwidth of high-brightness LEDs is sufficient to ensuretemporal coherence for holographic reconstruction. A line light sourcecan be considered a point light source if seen from a right angle to itslength. The light is then coherent in this direction and incoherent inthe perpendicular direction. In order to ensure temporal coherence, thelight must have an adequately narrow wavelength range. Color hologramscan be displayed when the information may be divided spatially intospectral portions monochromatically, sequentially or by way of filtermeans. The electronically controllable pixels arranged in theholographic array can be an intensity-modulating SLM, a phase-modulatingSLM or an SLM that modulates both the amplitude and the phase of thelight capable of generating interference. Pixel arrays, which are unableto directly control the phase of the coherent light, like a conventionalLCD, may use the known detour phase coding method so that amplitudesettings with several controllable pixels per holographic image pointcontrol the light phase. For encoding a complex value for a singleholographic image point of the array such known encoding technique usesthree electronically controllable pixels.

In contrast to common known solutions, the present invention and thesolution according to application WO 2004/044659 (A2) encodes thehologram information of a single scene point to a restricted encodedarea of the holographic array only. The extension and position of theencoded area are chosen such that the reconstructed scene point isvisible only within the viewing window. The observer cannot see theperiodic repetitions of the reconstructed scene point, as the lightemanating from those points does not reach the central viewing window.The extension and position of the encoded area depend on the x, y, and zcoordinates of the scene points.

The pupil of each eye has to be located in the viewing window. Due tothe smallness of the window, an eye position-tracking device detects theobserver's eyes and controls the position of the viewing windowaccording to the observer's movement. Vertical tracking is achieved byvertical shifting of the light source. This will shift the viewingwindow containing the reconstructed scene.

Another way to reduce the expense of reconstruction video holograms isto assign two separate viewing windows, each to one eye of the observer,achieved by two separate, adequately coherent light sources beingalternately turned on and two separate holograms encoded synchronouslywith the switch-over of the light sources. The SLM alternately encodesthe two video holograms displaying different perspective views. Due tothe low refresh frequency and long switching delays of availablehardware, a sequential representation leads however to cross talkbetween the two eyes.

SUMMARY OF THE INVENTION

It is an object of this invention to reconstruct video holograms inreal-time using electronically controlled pixels arranged in an arraywith a minimal resolution (e.g. commercially available at reasonablecost for the mass market), a minimum of flickering and cross talk;reconstruction is simultaneous for both eyes over a large viewing area,so that the required refresh rate is substantially low.

In order to achieve this object, the present invention is based on amethod for encoding and reconstructing video holograms where coherentlight supplied by a line light source and imaged by optical focusingmeans passes through controllable pixels of a single holographic arrayinto viewing windows, which are located in one diffraction order nearthe eyes of an observer, in order to reconstruct a scene holographicallyencoded by the controllable pixels and thus to render it visible throughtwo separate viewing windows for the observer's two eyes.

According to the present invention, the light used is only coherent inthe vertical direction, so that the controllable pixels generate avertical, one-dimensional reconstruction of holograms of the same 3Dscene for both eyes of the observer. The encoding of the holographicarray is horizontally split into two groups of pixel columns. Each groupis assigned to one of two separate video holograms: each displays one ofthe two perspective views (one for each eye). The holographic array isencoded with both groups of pixel columns horizontally interleaved atthe same time; thereby two spatially interleaved holograms of a sceneare generated. This means that all pixel columns of a first column groupof the pixel array reconstruct a hologram for one eye of an observer,while the adjacent pixel columns of the second column group reconstructthe hologram for the other eye at the same time. Since the horizontallyincoherent light reconstructs the scene in a conventional way, knownimage separation means with separating elements arranged parallel to thecolumns can be used to select the two holograms for the left and righteye. The image separation means is disposed at a distance to the pixelarray and reveals the column group for one eye and covers the columngroup for the other eye.

It is further an object of the invention to provide a device forencoding and reconstructing video holograms using a flat and thinhousing and that avoids costly and heavy optical and electronic hardwarecomponents with large physical dimensions.

The device according to the present invention is based on a line lightsource which provides coherent light and which consists of verticalfocusing means and a pixel array, which contains controllable pixels,which modulate amplitude or phase or both parameters of the coherentlight.

According to the present invention, the light source is arrangedhorizontally, so that its light is coherent in the vertical directionand incoherent in the horizontal direction. One-dimensional videoholograms are encoded by controllable pixels of the pixel array in pixelcolumns so that a first and a second column group separately encode aone-dimensional, vertically diffracting hologram of the same scene forthe two eye positions, where both column groups are interleavedhorizontally. The pixel columns are interleaved in such a way that theimage separation means, which is in the optical path of the light,exposes or reveals the column groups for one eye of the observer, andcovers it for the other eye with the help of separating elementsarranged parallel to the gaps. A hologram for each eye reconstructs thethree-dimensional scene in the reconstruction space in front of eachrespective viewing window. The two holograms differ in a horizontalparallax according to the observer's eye distance. The twoholographically encoded column groups represent two holographicreconstructions of the same scene. Both eyes see these holograms at thesame time in separate viewing windows.

A barrier mask may be disposed at a distance to the pixel array as animage separation means. The separating elements are then transparent andnon-transparent stripes which always reveal one of the interleavedcolumn groups for the left or right eye of an observer and which coverthe ones for the other eye.

The holographically encoded columns of a group preferably reconstructpartial images of the same scene according to the eye position: thepartial images are composed to form a total reconstruction of the scenewhen viewed with both eyes.

The order of execution of the above-mentioned process steps may vary. Inparticular, the steps of focusing, holographic modulation and hologramseparation may be swapped.

In order to make available the viewing windows in a large zone for theobserver, an eye position detection system tracks the horizontal,vertical and preferably also the axial position of observer's eyes sothat the position of the viewing windows can follow accordingly if theobserver moves. Alternatively, the encoding device can change orre-calculate the encoded hologram from a different perspective viewbased on the new observer position, so that the reconstructions appearfor the observer displaced and/or turned, i.e. seen from a differentangle. Depending on the eye positions, the pixel array may be re-encodedwith the help of software so that the holographic reconstruction becomesvisible at a fixed spatial position.

If the vertical positions of the observer's eyes change, the viewingwindows are tracked by vertical displacement of the light source.

If the horizontal positions of the observer's eyes change, the viewingwindows are preferably tracked by displacing the column groups inrelation to the image separation means. Alternatively, the imageseparation means, and in particular the separating elements, may bedisplaced in relation to the column groups. The use of horizontal linelight sources facilitates viewing window tracking.

If the positions of the observer's eyes change axially, the distancebetween light source and optical focusing means will be adaptedaccordingly.

Tracking the viewing windows according to the positions of theobserver's eyes in front of the display ensures that the holographicreconstruction is visible in a large zone at continuously highreconstruction quality.

Now, the method and device according to the present invention will bedescribed in more detail with the help of an embodiment. The principleof the invention will be explained based on a holographic reconstructionwith monochromatic light. However, those skilled in the art willappreciate that this invention may as well be applied to colorholographic reconstructions. In the latter case the controllable pixelsin each pixel column represent the primary colors required for a colorreconstruction in a spatially or temporally multiplexed way. The presentinvention therefore enables full colour, full motion holographictelevision, films, computer games and specialist applications benefitingfrom 3D video reproduction.

These and other features of the invention will be more fully understoodby reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a reconstruction of a 3D scene with two bundles of raysbetween a viewing plane and image separation means (top view).

FIG. 2 shows a detail of the device according to the invention(projection).

FIG. 3 shows a detail of the device according to the invention (topview).

FIG. 4 shows a reconstruction of a scene, which can be perceived by theobserver (top view).

DETAILED DESCRIPTION OF THE INVENTION

During the course of this description, like numbers will be used toidentify like elements according to the different views that illustratethe invention.

In the preferred embodiment of the invention, the information about athree-dimensional scene is encoded in a transmissive holographic array,where the pixels are computer controlled according to the encoding toform a pixel pattern. However, the basic idea of the invention is notlimited to the described transmissive holographic array. Bothtransflective and reflective holographic arrays or arrays that directlymodulate the phase of the light waves, such as Freedericksz cells, maybe used.

FIG. 1 shows a line light source 1 that illuminates, via focusing means2, a holographic array 3. The focusing means 2 is a horizontallydisposed cylindrical lens that images the light of the line light source1 into a viewing plane 5 containing viewing windows 8 r, 8 l.

FIG. 1 relates to a holographic encoding of a single small object; theholographic array reconstructs the 3D scene to be displayed. The smallreconstructed object 11 is shown by a circle. According to theinvention, the holographic information of the reconstructed object 11 isencoded in two restricted areas AL and AR of the holographic array 3only. The extent and positions of the encoded areas AL, AR are chosensuch that the reconstructed object 11 is visible only within thecorresponding viewing windows 8 r or 8 l.

For reconstructing the object 11, the holographic array 4 is encodedwith a pixel pattern that forms bundles of light rays 9 and 10, beingspatially light modulated by passing through the controllabletransmissive pixels of the holographic array 3.

According to the present invention, the horizontally disposed line lightsource 1 generates light that is spatially coherent in the verticaldirection and incoherent in the horizontal direction due to its linearorientation. On the pixels of holographic array 3, patterns of twodifferent video holograms are encoded, which are arranged in columns;the columns only reconstruct in the vertical direction. The hologramscan be seen in a diffraction order through a viewing window 8 r, 8 l inthe viewing plane 5. Due to the incoherence of the light in thehorizontal direction, the light source 1 is only imaged by focusingmeans 2.

FIG. 1 shows further an image separation means 7 making the bundle ofrays 9 for the right eye R visible and a bundle of rays 10 for the lefteye L.

Because the focusing means 2 and the holographic array 3 are disposed ata minimum distance to each other, their position may even be swapped.The image separation means 7 with its separating elements in the opticalpath may also be disposed at various positions.

The optical focusing means 2 may be a vertically focusing cylindricallens as shown, a Fresnel lens or a lenticular.

The holographic array 3 corresponds with that of a high-resolutiontransmissive flat display, e.g. a conventional LCD. If the detour phasemethod (or a similar method that modulates the amplitude of the light)is used for setting the phase relation, a higher diffraction order in aperiodicity interval is used for holographic reconstruction. Further,the pixels needed for phase control must be disposed next to each otherin the vertical direction, because the light is not capable of creatinginterference in the horizontal direction. Pixel arrays typically used inflat displays have sub-pixels disposed next to each other in thehorizontal direction; this is why such arrays must be turned by 90°.Pixel arrays with pixels which directly modulate both amplitude andphase of the light, may preferably be used, such as light modulatorsbased on Freedericksz cells.

FIG. 2 shows the device according to the invention in a projection view,and FIG. 3 shows the top view of a detail. The vertically coherentillumination of each hologram results in a reconstruction 11 of thethree-dimensional scene that is generated in frustums 12 r, 12 l thatstretch between the edges of the pixel array 3 and the viewing windows 8r, 8 l. In contrast to FIG. 1, FIG. 2 shows a holographic reconstruction11 of a complete 3D scene. The reconstruction 11 may be located in frontof, on or behind the holographic array 3. The horizontally incoherentillumination allows known image separation means to be used to selectthe holograms for each respective eye, so that a reconstruction of thescene with horizontal parallax is generated by the two holograms whenviewed with both eyes. This means that both holograms differ in ahorizontal parallax according to the eye distance. One-dimensional videoholograms are encoded in controllable pixels of the pixel array 3arranged in pixel columns 15, 15′ to 16″, where the column group 15, 15′and 15″ reconstructs a one-dimensional hologram for the right eye R ofthe observer, and the column group 16, 16′ and 16″ reconstructs aone-dimensional hologram of the same scene for the left eye L of theobserver. In the described embodiment, the two column groups arealternately interleaved in the horizontal direction. The pixel columns15 to 16″ are interleaved in such a way that the image separation means7 (which is in the optical path of the light and which consists ofseparating elements 17, 18, 19 arranged in line with the pixel columns15 to 16″) reveals the column group 15, 15′ and 15″ for one eye of theobserver and covers the column group 16, 16′ and 16″, respectively, forthe other eye.

The spatially interleaved holograms may be separated with the help of abarrier mask having vertical, alternately transparent andnon-transparent stripes 17, 18, 19, the mask being disposed in front ofor behind the pixel array 3. The transparent stripes reveal the firstcolumn group 15, 15′ and 15″ for the right eye R. The first column groupis at the same time covered by the non-transparent stripes for the lefteye L1. In FIG. 2, the arrows 20 and 21 show the view of the observerfrom the tracked, virtual viewing windows 8 r, 8 l in the viewing plane5 to the pixel columns 15 and 16 using the simplified example of asingle column 15, 16 of each column group. In reality, several hundredsof pixel columns and pixel column groups contribute to the interleavedholographic reconstructions of the three-dimensional scene 11. The imageseparation means contains as many separating elements.

The distance between the image separating mask 7 and the pixel array 3(see FIG. 4) and the distance between the individual stripes, which arepreferably of the same width, is chosen so that only the video hologramsintended for one eye are visible for the observer.

According to a preferred embodiment of the present invention, severaladjacent pixel columns may be grouped to form a common multiple pixelcolumn. Each multiple pixel column belongs to one of the two hologramsand is rendered visible as a whole hologram for the respective eye R orL by the image separation means 7. In this case, the multiple pixelcolumns of the two holograms interleave alternately instead of thesingle pixel columns.

Encoding holograms in multiple pixel columns reduces cross-talkingeffects. If the observer moves horizontally, full tracking of theviewing windows can effectively suppress image distortions. The pixelcolumns of the multiple pixel columns preferably modulate the lightdifferently. For example, only one pixel column of two or more adjacentpixel columns of each multiple pixel column actively modulates thelight, whereas the other one(s) are inactive. This means they are turnedoff The movement range without cross talk from the hologram for theother eye increases accordingly.

This results in a desired zone between the two viewing windows in whichno reconstruction is visible. Without this measure, an observer wouldsee a distorted reproduction in this zone until the viewing window istracked. Viewing window tracking in the horizontal direction can only beaccelerated if the position detection system activates and deactivatesindividual pixel columns within multiple pixel columns when the observermoves horizontally.

The image separating means may be a barrier mask, a lenticular designedfor this purpose or a prism mask. The image separating elements of apreferred embodiment has about twice the pitch of the correspondingcolumn group of the pixel array.

A position detection system (not shown) is used to track the position ofthe viewing windows.

Changes in the horizontal eye position can be tracked by displacing theseparating elements of the image separation means 7. This can be doneelectronically, for example with the help of another transmissive pixelarray with controllable openings.

Alternatively, the column groups 15 to 16′ can be displaced horizontallyin relation to the image separation means 7 to track the viewing windows8 r, 8 l. This may also be done electronically, preferably with the helpof above-mentioned multiple pixel columns. If only one pixel column ofeach multiple pixel column is active at a time to modulate the light,switching over to a different pixel column of a multiple pixel columncan support horizontal tracking.

Further, both horizontal displacement methods may be carried outsimultaneously. The viewing windows may be tracked in the verticaldirection by vertically displacing the line light source 1. Changing thedistance between the light source 1 and the cylindrical lens 2 maycompensate for an axial position change.

If an observer moves, not only will the viewing windows be tracked. Thecontents of the holograms may also be recalculated and re-encodedaccordingly to take into account the different perspective.

A distinctive feature of the solution proposed by this invention is thatfor each of the observer's eyes R, L a corresponding video hologram iscalculated, encoded and reconstructed at the same time, and that theseholograms are spatially interleaved using a single pixel array 3, andthat these two holographic reconstructions may be rendered visibleseparately in the respective viewing windows 8R, 8L for the right andleft eye using the image separation means 7.

The two video holograms are characterized by a horizontal parallax thatcorresponds to the eye distance. This ensures a true 3D impression ofthe scene.

The simultaneous holographic reconstruction of the scene for both of theobserver's eyes guarantees a natural view, so that the accommodation andconvergence of the observer's eyes on any point of the scene is ensured,given correct focusing.

The vertical dimension of the viewing window is within one diffractionorder and should not exceed the periodicity interval of thereconstruction of the video hologram. Otherwise the observer will see asuperposition of the reconstruction of two adjacent diffraction orders.Further, the size of the viewing windows must be adapted to thepositioning precision and the tracking speed. A deliberate narrowing ofthe vertical size of the viewing window for encoding, as known from WO2003/021363 (A1), for example to 10 mm, reduces the required displayresolution and the volume of data to be processed and transmitted atleast by a factor of 100.

The present invention makes it possible to use commercially availablecontrollable matrices, e.g. LCD flat displays, for hologramreconstructions. In case of color encoding, the alternate RGB sub-pixelsfor the three primary colors reconstruct three individual partialholograms of a primary color, which are then composed to form a colorreconstruction.

FIG. 4 shows a device according to the invention with all superimposedbundles of rays 13 for the right eye and bundles of rays 14 for the lefteye.

A major benefit of the present invention is that the display is run in aspace-multiplex mode at double the image frequency compared to atime-multiplex mode. Thereby, two spatially interleaved holographicreconstructions of a three-dimensional scene are encoded in one pixelarray, but are seen separately by the observer thanks to the imageseparation. This method allows halving of the image frequency.

The viewing windows 8R, 8L must have a size of at least the dimension ofthe eye pupil so that the observer is able to view a flawlessholographic reconstruction. However, this minimum size would makeextreme demands on the tracking precision and speed, which it may beunrealistic to achieve, so that in practice the viewing windows must beconsiderably larger. However, the height of the viewing window cannotexceed the extent of the periodicity interval. Further, it is necessaryto adjust the width of the viewing windows. Based on the width of thepixel columns, this is achieved by matching the lateral extent with thedimensions of the image separation means, in particular with the pitchand width of the separating elements. The two viewing windows aredisposed with their centers about an eye distance apart.

The use of one-dimensional, vertically reconstructing holograms, inconjunction with horizontally incoherent light, for scene reconstructionconsiderably reduces the computing power required for the provision ofencoding data. Also the demands on the resolution of the pixel array inthe horizontal direction are not critical, so that large-area videoholograms can be reconstructed with little effort in a large viewingzone, thus allowing the eye positions of an observer to move.

In contrast to point light sources, the line light source used allows acontinuous reconstruction even with lateral movement of the observer.

Another advantage of the present invention is that a conventional whitelight source can be used in combination with a slot mask, instead of alaser.

The invention is suitable for both the entertainment sector, i.e. forTV, multimedia, game stations and mobile telephone terminals, and forcommercial applications such as 3D CAD, in medical and militaryequipment, as well as many other appliances involving displays.

While the invention has been described with reference to the preferredembodiment thereof, it will be appreciated by those of ordinary skill inthe art that modifications may be made to the parts that comprise theinvention without departing from the spirit and scope thereof.

1. Method for encoding video holograms for holographicallyreconstructing a scene where optically vertical focusing means imagevertically coherent light emitted by a line light source and modulatedby electronically controllable pixels of a holographic array into aviewing plane with viewing windows in order to holographicallyreconstruct a three-dimensional scene within one diffraction order byway of diffraction at the pixels and to render this scene visible fromthe viewing windows for both eyes of an observer, wherein thecontrollable pixels encode two separate holograms of left or right eyeperspective views of the same scene in separate pixel columns one foreach of the viewer's eyes said holograms being one-dimensional in thevertical direction, where the pixels are arranged in separate columngroups so that the two one-dimensional holograms are horizontallyinterleaved, and an image separation means with separating elementsarranged parallel to the pixel columns reveals the respective pixelcolumns for one eye and covers them for the other eye.
 2. Methodaccording to claim 1 where the holograms for both of the observer's eyesencode a horizontal parallax according to the eye to eye separationdistance.
 3. Method according to claim 1 where local changes in theobserver's eye positions are tracked by an eye position tracking system.4. Method according to claim 3 where the holographic encoding of thepixels in the pixel columns is updated according to a change in eyepositions.
 5. Method according to claim 4 where the reconstructed sceneis encoded so that it will appear for the observer at a fixed horizontalor vertical or axial position as the eye positions change.
 6. Methodaccording to claim 4 where the reconstructed scene is encoded so that itappears for the observer horizontally or vertically displaced or turneddepending on a horizontal or vertical or axial change in the eyepositions.
 7. Method according to claim 3 where the viewing windows aretracked by vertically displacing the line light source according tovertical change of the eye positions.
 8. Method according to claim 3where the viewing windows are tracked by horizontally displacing thecolumn groups in relation to the image separation means according tohorizontal change of the eye positions.
 9. Method according to claim 3where the viewing windows are tracked by horizontally displacing theseparating elements of the image separation means in relation to thepixel columns according to horizontal change of the eye positions. 10.Method according to claim 3 where the distance between the light sourceand the optical focusing means is adapted accordingly according to axialchange of the eye positions.
 11. Device for encoding video holograms forholographically reconstructing a scene, comprising a line light sourceemitting light, which is coherent in one direction and an opticalfocusing means in order to holographically reconstruct a scene infrustum-shaped reconstruction spaces after modulating the light bycontrollable pixels arranged in a holographic array characterized by theline light source being arranged horizontally so that its light showsadequate coherence in the vertical direction, the controllable pixelsare arranged in pixel columns with one column group for each of aviewer's and contain one-dimensional, vertically encoded holograms ofleft or right eye perspective views of the same scene, where both columngroups are interleaved horizontally, and an image separation means withseparating elements arranged parallel to the pixel columns that revealsthe pixel columns for one eye and covers them for the other eye. 12.Device according to claim 11 where several adjoining pixel columns eachare combined to form common multiple pixel columns, where every multiplepixel column is selectable as a whole for the respective eye by theimage separating means in order to suppress cross-talking between thereconstructions of the holograms or to adjust the size of the viewingwindows.
 13. Device according to claim 12 where the pixel columns of amultiple pixel column modulate the light differently.
 14. Deviceaccording to claim 11 where the pixels are transmissive and/orreflective and modulate the amplitude or optical phase of the coherentlight.
 15. Device according to claim 11 where the optical focusing meansis a vertically focusing cylindrical lens, Fresnel lens or lenticular.16. Device according to claim 11 where the image separation means is abarrier mask, lenticular or prism mask.
 17. Device according to claim 11where the image separation means is a barrier mask with transparentstripes which reveal the column groups intended for one eye of theviewer and with non-transparent stripes which cover the other columngroups for that eye.
 18. Device according to claim 16 where the pitch ofthe separating elements of the barrier mask the prism mask or thelenticular is about twice the horizontal pitch of the correspondingcolumn group of the controllable pixel array.
 19. Device according toclaim 11 where the controllable pixels further comprise sub-pixels forcolour reconstruction and are disposed one below the other verticallyfor detour phase encoding.
 20. Device according to claim 12 where theviewing windows have at least the size of a pupil, are defined by thewidth of the image separation means in horizontal direction for eacheye, and are such that the centers of the two windows are about aninterocular distance apart.
 21. Device according to claim 12 where theviewing windows for the two eyes have a vertical extension which is notgreater than the periodicity interval of one diffraction order but whichis not smaller than a pupil.